linux-kernel - Re: [RFC PATCH] iommu/vt-d: Fix IOMMU field not populated on device hot re-plug

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Date:   Thu, 29 Aug 2019 17:08:18 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Michał Wajdeczko <michal.wajdeczko@...el.com>
Subject: Re: [RFC PATCH] iommu/vt-d: Fix IOMMU field not populated on device
 hot re-plug

Hi,

On 8/29/19 3:58 PM, Janusz Krzysztofik wrote:
> Hi Baolu,
> 
> On Thursday, August 29, 2019 3:43:31 AM CEST Lu Baolu wrote:
>> Hi Janusz,
>>
>> On 8/28/19 10:17 PM, Janusz Krzysztofik wrote:
>>>> We should avoid kernel panic when a intel_unmap() is called against
>>>> a non-existent domain.
>>> Does that mean you suggest to replace
>>> 	BUG_ON(!domain);
>>> with something like
>>> 	if (WARN_ON(!domain))
>>> 		return;
>>> and to not care of orphaned mappings left allocated?  Is there a way to
> inform
>>> users that their active DMA mappings are no longer valid and they
> shouldn't
>>> call dma_unmap_*()?
>>>
>>>> But we shouldn't expect the IOMMU driver not
>>>> cleaning up the domain info when a device remove notification comes and
>>>> wait until all file descriptors being closed, right?
>>> Shouldn't then the IOMMU driver take care of cleaning up resources still
>>> allocated on device remove before it invalidates and forgets their
> pointers?
>>>
>>
>> You are right. We need to wait until all allocated resources (iova and
>> mappings) to be released.
>>
>> How about registering a callback for BUS_NOTIFY_UNBOUND_DRIVER, and
>> removing the domain info when the driver detachment completes?
> 
> Device core calls BUS_NOTIFY_UNBOUND_DRIVER on each driver unbind, regardless
> of a device being removed or not.  As long as the device is not unplugged and
> the BUS_NOTIFY_REMOVED_DEVICE notification not generated, an unbound driver is
> not a problem here.
> Morever, BUS_NOTIFY_UNBOUND_DRIVER  is called even before
> BUS_NOTIFY_REMOVED_DEVICE so that wouldn't help anyway.
> Last but not least, bus events are independent of the IOMMU driver use via
> DMA-API it exposes.

Fair enough.

> 
> If keeping data for unplugged devices and reusing it on device re-plug is not
> acceptable then maybe the IOMMU driver should perform reference counting of
> its internal resources occupied by DMA-API users and perform cleanups on last
> release?

I am not saying that keeping data is not acceptable. I just want to
check whether there are any other solutions.

Best regards,
Baolu